A planar lightwave circuit (PLC) is the optical equivalent of an electronic chip, manipulating and processing light signals rather than electronic signals. In most cases, a PLC is formed in a relatively thin layer of glass, polymer or semiconductor formed on a semiconductor substrate. The lightwave circuit itself is composed of one or more optical devices interconnected by optical waveguides, the waveguides functioning to guide the light from one optical device to another and therefore considered as the optical equivalent of the metal interconnections in an electronic chip. The optical devices may comprise either passive optical devices, or active electro-optic devices, performing functions including, for example, reflection, focusing, collimating, beam splitting, wavelength multiplexing/demultiplexing, switching modulation and detection, and the like.
As of now, the common planar optical devices formed in a silicon-on-insulator (SOI) structure use a relatively thick (>3–4 μm) silicon surface waveguiding layer (hereinafter referred to as an “SOI layer”), so as to allow for relative ease of input/output coupling of a light signal to the SOI layer through methods such as edge illumination. However, the need for edge-illumination coupling requires access to the edge of the chip, as well as the formation of an edge with a high surface quality (to prevent reflections and backscattering at the coupling interface). Further, the fabrication of high definition structures is considered to be rather difficult in a relatively thick SOI layer (for example, forming “smooth” vertical sidewalls for waveguides, rings, mirrors, etc.). The thickness of the silicon also prevents the use of conventional CMOS fabrication processes to simultaneously form both the electronic and optical components. Additionally, optical structures formed in such a relatively thick layer will tend to support the propagation of multiple optical modes, complicating the ability to provide transmission of only the fundamental mode through the structure. Moreover, the relatively large thickness of the SOI layer also limits the speed of the electronic devices.
Once the thickness of the SOI layer is limited to being less than one micron, most of the above-described problems are significantly reduced, if not completely eliminated. However, in order to form a viable photonic circuit in a sub-micron thick SOI layer, the need remains to be able to actively manipulate the light within the SOI layer. More particularly, the need exists to perform various optical functions, such as turning, focusing, modulating, attenuating, deflecting, switching and selectively dispersing the light propagating sub-micron thick SOI layer, preferably utilizing conventional CMOS-compatible voltage levels to accomplish the active manipulation so that a common power supply may be used to control optical and electrical components formed on a common silicon substrate.